Arimori was born into the Tsuchimikado house as the son of the thirty ninth head Exorcist Arima Tsuchimikado and his wife Mana. As a young child, he was very close to his parents and sought to be a powerful Exorcist like his father. At some point, he skipped elementary school and enrolled in the Seiyouin Exorcist Academy. One day, his mother died and he vowed to be a strong exorcist like his father and aid him in his duties. However, as he trained he struggled with trying to impress his father who became cold in his grief and bluntly asked Arimori if he was really had the talent to be his son causing the latter to break down. Arimori later joins the Enmadou house.